2.8. Analysis of Active Compounds of CFE from LAB Strain Inhibiting DPP-4

The samples with a DPP-4 inhibitory activity were fractionated using ultrafiltration tubes (Millipore Corporation, Bedford, MA, USA) with molecular mass cutoffs of 3, 5, 10, and 30 kDa. A 30-kDa ultrafiltration tube was first used to divide the sample into >30 kDa and <30 kDa, and then a 10 kDa ultrafiltration tube was used to divide the <30 kDa sample into 10–30 kDa and <10 kDa. Similarly, 5 kDa and 3 kDa ultrafiltration tubes were successively used for additional gradual separation. Finally, the collected fractions were lyophilized and preserved at −80 °C. The <3 kDa fractions of CFE from LAB strain inhibiting DPP-4 were further analyzed by UHPLC-MS/MS using a method described in a previous study [22].